CSE331:
Introduction to Networks
and Security
Lecture 8
Fall 2002
Announcements
• HW1 is graded
– Pick up after class
– Mean = 81
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Recap
• Spanning tree algorithms
– Finds routing information for bridges
– Doesn’t scale to Internet size networks
• Internet Protocol (IPv4)
– Packet format
– Packet fragmentation
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Today
• IP Addressing
• Subnetting
• DNS (Domain Name System)
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IP addresses
• Hierarchical, not flat as in Ethernet
0
7
24
Network
Host
1 0
14
16
Network
Host
21
1 1 0
A
B
8
Network
Host
C
• Written as four decimal numbers separated
by dots: 158.130.14.2
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Network Classes
Class # of nets
A
126
# of hosts
per net
~16 million
B
8192
65534
C
~2 million 254
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IP Datagram Delivery
• Every IP packet (datagram) contains the
destination IP address
• The network part of the address uniquely
identifies a single network that is part of the
larger Internet.
• All hosts and routers that share the same
network part of their address are connected
to the same physical network.
• Routers can exchange packets on any
network they’re attached to.
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IP Forwarding algorithm
• If (Network # dest == Network # interface)
then deliver to destination over interface
• else if (Network # dest in forwarding table)
deliver packet to NextHop router
• else deliver packet to default router
• Forwarding tables
– Contain (Network #, NextHop) pairs
– Additional information
– Built by routing protocol
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Question:
• How does the hierarchical address scheme
improve scalability?
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ARP - Address Resolution Protocol
• Problem:
– Need mapping between IP and link layer addresses.
• Solution: ARP
– Every host maintains IP–Link layer mapping table
(cache)
– Timeout associated with cached info (15 min.)
• Sender
– Broadcasts “Who is IP addr X?”
– Broadcast message includes sender’s IP & Link Layer
address
• Receivers
– Any host with sender in cache “refreshes” time-out
– Host with IP address X replies “IP X is Link Layer Y”
– Target host adds sender (if not already in cache)
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ICMP: Internet Control Message Protocol
• Collection of error & control messages
• Sent back to the source when Router or Host
cannot process packet correctly
• Error Examples:
–
–
–
–
Destination host unreachable
Reassembly process failed
TTL reached 0
IP Header Checksum failed
• Control Example:
– Redirect – tells source about a better route
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Scaling Problems
• Not enough network numbers.
– Class C network with 2 nodes wastes 253 IP
addresses
– Class B network with ~300 nodes wastes 64,000
IP addresses
– Only 214 ~ 16,500 class B networks
• Routing information too cumbersome.
– More networks means larger routing tables
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http://www.cheswick.com/map/index.html
Subnetting
• Idea: One IP network number allocated to
several physical networks.
– The multiple physical networks are called subnets
– Should be close together (why?)
– Useful when a large company (or university!) has
many physical networks.
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Subnet Numbers
• Solution: Subnetting
– All nodes are configured with subnet mask
– Allows definition of a subnet number
• All hosts on a physical subnetwork share the same
subnet number
Subnet Mask (255.255.255.0)
111111111111111111111111
00000000
Subnetted Address:
Network number
Subnet ID
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Host ID
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Example of Subnetting
Subnet mask: 255.255.255.128
Subnet #: 128.96.34.0
H1
128.96.34.15
128.96.34.1
R1
128.96.34.130
Subnet mask: 255.255.255.128
Subnet #: 128.96.34.128
128.96.34.129
R1
H2
128.96.34.139
128.96.33.1
Subnet mask: 255.255.255.0
H3
Subnet #: 128.96.33.0
128.96.33.14
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Subnets, continued
• Mask is bitwise-ANDed with address
• This is done at routers
• Router tables in this model:
– <Subnet #, Subnet Mask, NextHop>
• Subnetting allows a set of physical networks
to look like a single logical network from
elsewhere
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Forwarding Algorithm
D = destination IP address
for each forwarding table entry
(SubnetNumber, SubnetMask, NextHop)
D1 = SubnetMask & D
if D1 = SubnetNumber
if NextHop is an interface
deliver datagram directly to destination
else
deliver datagram to NextHop (router)
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Domain Name System
• System for mapping mnemonic names for
computers into IP addresses.
zeta.cis.upenn.edu
• Domain Hierarchy
• Name Servers
• Name Resolution
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158.130.12.244
19
Domain Name Hierarchy
edu
com
cornell … upenn
cis
seas
gov
cisco…yahoo
nasa … nsf
mil
org
net
arpa … navy …
wharton …
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Hierarchy of Name Servers
Root
Name Server
Cornell
Name Server
…
Upenn
Name Server
CIS
Name Server
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SEAS
Name Server
Wharton
Name Server
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Records on Name Servers
• < Name, Value, Type, Class >
• Types
–
–
–
–
A Host to address mappings
NS Name server address mappings
CNAME Aliases
MX Mail server mappings
• Class IN for IP addresses
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Name resolution
Root
Name
server
zeta.cis.upenn.edu
client
198.168.0.100
Local
Name
server
zeta.cis.upenn.edu
198.168.0.1
Upenn
Name
server
CIS
Name
server
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